Microwave moisture measurement

Abstract

A moisture content measuring system for sheet material which uses microwave radiation. Two pairs of microwave radiators and receivers are combined with surface and below surface temperature measuring sensors to furnish data to a computer which interprets the data and yields moisture readings. Each pair of microwave radiator and receiver straddles the sheet test sample and checks microwave transmission through the sample and reflected from it, but the two radiators are cross polarized so that signal interchange between them is avoided.

What is claimed is: 
     
       1. An apparatus for measuring the moisture content of sheet material comprising: sheet support means to locate a sheet to be tested in a particular plane;   microwave source means;   first radiator means connected to and receiving power from the microwave source means and radiating power transverse to one of the two planar surfaces of the test sheet;   first receiver means aligned with the first radiator means but located on the other side of the test sheet from the first radiator means;   second radiator means connected to and receiving power from the microwave source means and radiating power transverse to the second planar surface of the test sheet, and oriented so that its radiation is cross polarized relative to the radiation of the first radiator means;   second receiver means aligned with the second radiator means but located on the other side of the test sheet from the second radiator means;   first detector means, connected to the first radiator means, measuring the power applied to the test sheet by the first radiator means;   second detector means, connected to the first radiator means, measuring the power reflected from the test sheet back to the first radiator means;   third detector means, connected to the first receiver means, measuring the power from the first radiator means transmitted through the test sheet;   fourth detector means, connected to the second radiator means, measuring the power applied to the test sheet by the second radiator means;   fifth detector means, connected to the second radiator means, measuring the power reflected from the test sheet back to the second radiator means;   sixth detector means, connected to the second receiver means, measuring the power from the second radiator means transmitted through the test sheet;   first temperature measuring means located near one surface of the test sheet and measuring the temperature of one planar surface of the test sheet;   second temperature measuring means located near the second surface of the test sheet and measuring the temperature of the second planar surface of the test sheet;   third temperature measuring means located near one surface of the test sheet and measuring the temperature of the test sheet at a level between its two exterior surfaces; and   data acquisition means, connected to and receiving electrical signals from all six detector means and all three temperature measuring means, factoring the electrical signals received to produce a measurement of moisture content in the test sheet, and displaying the moisture content for the test sheet.   
     
     
       2. The moisture content measuring apparatus of claim 1 wherein the data acquisition means is a computer. 
     
     
       3. The moisture content measuring apparatus of claim 1 further including a microwave phase adjustment means inserted between the microwave source and the first radiator means to permit maximization of the power applied to the test sheet. 
     
     
       4. The moisture content measuring apparatus of claim 1 further including a second phase adjustment means inserted between the microwave source and the second radiator means to permit maximization of the power applied to the test sheet. 
     
     
       5. The moisture content measuring apparatus of claim 1 wherein the first temperature measuring means is a passive infrared thermopile. 
     
     
       6. The moisture content measuring apparatus of claim 1 wherein the second temperature sensing means is a passive infrared thermopile.